What is PIM?

Discussion in 'The PIM Corner' started by PimTesting.com, Sep 27, 2011.

  1. PimTesting.com

    PimTesting.com Industry Observer

    To help get the forum going, below is the first of a serious of short articles written to help clarify the issues and misinformation about PIM and PIM Testing.

    Article 1: What is PIM?

    What is PIM? We’d like to say that it depends who you ask, but the fact is that PIM has a simple definition. PIM is an acronym for Passive Inter-Modulation. Inter-modulation refers to a series of unwanted (but related) frequencies created whenever two or more frequencies pass through a nonlinear device or junction. Where the confusion comes in is that depending on who you ask, they may have a different perspective about what they need to do about PIM, and that changes how they think of it.

    For the engineers, the short and sweet definition is all they need. Whenever two or more frequencies (in our case, transmit frequencies) pass through a nonlinear device, the desired frequencies are mixed. This creates a series of related frequencies, called harmonics, which are mathematically related to the desired input frequencies and are collectively referred to as PIM. Engineers use this technical perspective of the definition to determine how to design better components and other systems to help minimize this effect.

    For the carriers and the BTS, it’s more important to know what those unwanted frequencies mean. So, for the carriers and the BTS, PIM represents noise or interference. Noise plays a critical role in BTS performance -- the noise level (or noise floor) largely determines BTS receiver performance. The lower the noise floor, the better the BTS performance. Since the BTS sees PIM as noise, higher PIM levels increase noise levels. This is particularly true for wide-band technologies like CDMA, LTE and UMTS. Lower PIM levels mean better, more efficient BTS operation, which is why PIM has become so important to the wireless carriers in recent years.

    Finally, for installers, technicians, and others out in the field, PIM is a measure of installation and manufacturing quality. The leading causes of signal noise are poor construction and poor component and/or installation quality. So, it makes sense: if PIM is noise, it is reasonable to conclude that lower PIM levels indicate higher installation quality. When a site is PIM tested, two test tones are used to generate a known PIM signal (called IM3). The PIM test set is tuned to accurately measure IM3, and the lower the PIM level, the higher the component and installation quality.

    Although Engineers, Carriers and Installers may interpret the definition of PIM differently, each group is correct. PIM is a series of unwanted-but-related frequencies that occur when two or more frequencies pass through a nonlinear device or junction. PIM is noise. PIM is interference. And, PIM is a quality indicator. Regardless of which definition makes the most sense for you, it’s important to understand that all three views are correct.

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